* Argentina bars corn export sales through February

* Dry weather threatens crops in Argentina, southern Brazil

* Corn posts biggest annual gain in a decade

* Soybean yearly gain largest since 2007

CHICAGO, Dec 31 (Reuters) - U.S. corn futures closed their strongest year in a decade on Thursday as Argentina curbed its exports and dry weather continued to threaten harvests in South America, sending prices to their highest in 6-1/2 years.

Soybeans also scaled a 6-1/2-year peak as South American weather worries and tightening global supplies fueled a rally punctuating the oilseed's strongest yearly price gain since 2007.

Wheat finished 2020 in the black for the fourth straight year after touching a six-year high on Thursday and ending mostly firm.

After climbing for 14 straight sessions, corn ended up 24.8% for the year. Soybean prices climbed 37.2% from a year ago, while wheat added 14.6%.

Corn guided grains higher on Thursday after Argentina's agriculture ministry announced that the country would suspend sales of corn for exports until Feb. 28 to ensure ample domestic food supplies.

The move triggered more buying in a futures market already on edge due to crop-threatening South American weather amid robust demand and tightening global corn and soy supplies.

"There is continued concern about Argentine and southern Brazilian weather," said Jack Scoville, analyst with the Price Group. "There's also a lot of fund buying just based on the trends, which are extremely up."

Argentina and southern Brazil are expected to remain hot and dry for much of the next two weeks, according to meteorologists.

Chicago Board of Trade March corn jumped 9-1/2 cents to $4.84 a bushel, the highest for a most-active contract since May 16, 2014.

March soybeans gained 10-1/2 cents to $13.11 a bushel, the loftiest level for a most-active contract since June 23, 2014.

CBOT March wheat was down 1/4 cent at $6.40-1/2 a bushel after peaking earlier at a six-year high of $6.44-1/2.

All soybean contracts and most corn and wheat contracts hit life-of-contract highs. (Additional reporting by Emily Chow in Shanghai; Editing by Subhranshu Sahu, Vinay Dwivedi, Jonathan Oatis and David Gregorio)